Originally Posted by
kyselad
I don't know whether this problem is relatively recent or was already there at the time of manufacture, as it was already there when I bought the bike. It didn't have a rack or fenders when I bought it, so it's entirely possible the bike was built this way and whoever owned it previously just didn't notice.
It's an old bike. Things happen along the long lifetime of the bike. I wouldn't expect that it was made that way. It might be that it was "fixed" that way by someone after something worse happened. A slight bend in one of the stays from something or someone falling on it while it was laying on the ground might give enough to have it out of kilter.
How well does the rear axle fit in the slot of the drop out? If there is side to side play in the axle without the QR or nut being tightened, then does that account for the off-center between the seat stays? If so is the slot wallowed out? a crack developing in the radius of the slot, or the axle undersized?
You said you made something to check if the drops are parallel. They might be parallel, and the frame bent somewhere else. Still seems like if you have the drops held parallel to each other and you have that axis between them horizontal to the ground, then you would see if the frame leans one way or not. And if it does, then you can try to bend it back. Even if it bends something else.
It is after all an old frame. We don't have any frame builders anywhere near me that I'm aware of. Perhaps there might be some near you. If there are some that do it professionally for money there might be some that do it as a hobby. Perhaps you can find one of those willing to check out the frame alignment and fix it just for the sake of saving history. But for a one off thing, it'll take time. And time is money usually.